Natural gas has become more important for industries, such as power generation, to meet more stringent regulatory constraints, and provide cleaner alternatives to reduce equipment maintenance and service. As more infrastructure and fracking well supplies proliferate in the US and overseas markets, reliable traditional natural gas reserves are being mixed or replaced with natural gas supplies with more physical make-up variability. Natural gas supplies are becoming more readily available. Distribution sites and associated infrastructure, like ports, make products like compressed natural gas (CNG) and liquefied natural gas (LNG) more readily available to replace other fossil fuels such as coal and oil. However, as these traditional and new sources are mixed in pipelines and into saleable product, mixtures are becoming more varied and require more accurate and higher temporal resolution in continuous testing for undesirable contaminants if the company is going to meet consumer expectations.
Natural gas is composed primarily of methane (CH4) and hydrocarbon gases and contaminants such as carbon dioxide (CO2), nitrogen (N2), hydrogen sulfide (H2S) and water (H2O). As more sources of fracking gas come on-line and industries convert to cleaner burning fuels like CNG, the composition of CNG is more varied and includes species like sulfur compounds which may cause corrosion and performance issues.
The presence of sulfur in its most common reduced form in natural gas is as hydrogen sulfide (H2S). The hydrogen sulfide molecule consists of two hydrogen atoms bonded to a single sulfur atom. Its molecular composition is similar to that of water (H2O), two hydrogen atoms bonded to a single oxygen atom. Because sulfur can substitute as an oxidizer in place of oxygen, H2S is easily produced in anaerobic environments from the sulfur left over from decayed organic matter such as is found in fossil fuels. H2S is generally found in higher concentrations in fracking gas due to the association of the natural gas and H2S in the interstices of porous material where fracking gas is obtained. Fracking involves the recovery of shale gas which is natural gas that is trapped within layers and pores of rock formations or boundary layers. The fracking boom of the early 2000's has resulted in a variety of gas sources that differ in quality and physical characteristics such as sulfur content. Natural gas is considered “sour” if it has a high percentage of hydrogen sulfide. It has been estimated that 15 to 25% of natural gas in the U.S. may contain hydrogen sulfide at part per million by volume (ppmv) levels.
Energy companies must ensure that gas feeds from fracking meet quality industry standards before distributing to its customers. A fast and ultra-sensitive instrument for measuring H2S content is critical to the efficiency and cost effectiveness of these operations. Additionally, when one company sells or transfers gas to another company such as from a fracking site to a pipeline company, it is called a “custody transfer point.” At this point where the gas is transferred, the gas purchaser must measure hydrogen sulfide concentration or risk pipeline damage or damage to equipment at a downstream customer or consumer location. In most natural gas supply agreements, The Federal Energy Regulatory Commission (FERC) requires that pipeline gas contain hydrogen sulfide in amounts less than 4 PPM/V. Most custody transfer contracts also specify this limit.
Water is also found in natural gas and may be more concentrated in fracking gas due also to the association with the interstices in porous material and the need to inject large quantities of water in the fracking process. When water combines with carbon dioxide and hydrogen sulfide it forms carbonic and sulfuric acids which are very corrosive to metals. The acids can quickly break down metal pipes, tanks, or other metal materials with which they come into contact. The corrosive nature of H2S can cause pipes or tanks to develop leaks or weaken the structure, causing failure.
Thus, there is a need for an improved method and system for monitoring hydrogen sulfide, water, or other trace species in natural gas.